1. Field of the Invention
The present invention relates to an ink-jet image recording apparatus and, in particular, to an ink-jet image recording apparatus which records on the same recording area by using a plurality of different ink-jet heads for different recording operations.
2. Description of the Related Art
Ink-jet type image recording apparatuses are generally known as copying machines, printers, etc. Since the principle on which they record is a non-impact type, they are quiet and easily allow color recording. Nowadays, they are being achieving widespread use due to these advantages.
In a color image forming apparatus, which records images in a plurality of colors, a plurality of (for example, four) ink-jet heads are arranged in the scanning direction at predetermined intervals on a carriage that is adapted to move for scanning along a guide shaft oriented parallel, for example, to a sheet feeding roller, and inks of different colors, such as cyan, magenta, yellow and black, are ejected from the ink-jet heads to thereby form a color image on a recording sheet.
Generally speaking, a recording sheet used in an image recording apparatus of this type is required to have the ability to absorb ink quickly. In the case of a recording sheet that cannot absorb ink to a sufficient degree, when a large amount of ink is ejected onto the sheet over a relatively short time, it takes time for the ink on the sheet to permeate and be absorbed, so that some droplets of ink will be allowed to spread during that time on the sheet and join other droplets to grow into larger droplets of ink. In that case, the ink dots to be formed are not recorded at the proper positions and in the proper configuration, and a stripe-like pattern is generated on the entire image, so that the resultant image looks rather rough. This phenomenon is generally called beading. Further, when the absorption of ink is incomplete, the portion of ink which is not absorbed by the sheet but remains thereon will disturb the image to be formed or stain the interior of the recording apparatus.
To solve this problem, which is attributable to incomplete ink absorption, it is known to coat a recording sheet with a material which excels in ink absorptively.
However, in the case of a special type of recording sheet, there is a limitation in terms of the kind of material to be used for the coating, so, in some cases, such absorptively as can be obtained with an ordinary type of coated recording paper cannot be expected. For example, in an OHP sheet, the coating material must be transparent, so that a coating material used in ordinary recording papers, which contains silica as the main material, cannot be used. Further, while in the case of an ordinary coated recording paper, the paper portion constituting the base layer also absorbs ink, the transparent film of an OHP sheet does not absorb ink, so that the ink absorptively of an OHP sheet is inferior to that of ordinary recording papers. Thus, an OHP sheet is more liable to give rise to the problems described above.
To cope with these problems, the present applicant has proposed a recording method according to which, where recording is usually performed in a single scan by ejecting inks of the four colors of cyan, magenta, yellow and black, scanning is effected two times, recording being performed by ejecting cyan and black inks at the first scanning and magenta and yellow inks at the second scanning.
In this method, the amount of ink ejected onto the recording sheet in one scanning can be reduced by half, thereby making it possible to avoid the above problems. Although this method requires twice the recording time as the above arrangement of ink-jet heads, the ink-jet heads for cyan and black are most spaced away from each other, so that, even if the two inks are ejected during the same scan, the time elapsing between the ejection of cyan ink and that of black ink is relatively long, which mean beading is not so easily generated. In view of this, cyan and black inks are ejected during the same scan. As to which of the two pairs of colors is to be used first for scanning, it is expedient to use that pair first which is more liable to the generation of beading. This is due to the fact that, in some cases, beading can be generated easily when ink ejection is effected by using a pair of colors which is more liable to the generation of beading in a condition in which ink has already been ejected, that is, in a condition in which the absorptively of the sheet has been reduced.
In view of the above, in the double pass scanning mode, setting is effected such that cyan and black inks are ejected in the first scanning pass, and that magenta and yellow inks are ejected in the second scanning pass.
However, in some cases, the effect of the double pass scanning cannot be obtained to a sufficient degree with the above combination, depending upon the kind of image to be recorded. For example, when recording an image whose color is substantially only red, the above combination cannot provide the expected effect of the double pass scanning. To compared to the case in which inks of all the colors are ejected in a single scan, it proves substantially effective in terms of image quality. In view of this, it is expedient to provide a single-pass mode for high-speed recording and a double pass scanning mode for high image quality so that it is possible to select between the two modes according to the purpose, which arrangement enables the apparatus as a whole to be improved in terms of efficiency.
Generally speaking, in the case of the double pass scanning mode, the combination of the color of inks to be ejected in each scan is determined, taking various factors into consideration, such that the effect of the double pass scanning is maximum for average images. In some cases, however, when the combination is fixed, the effect of the double pass scanning cannot be obtained to a satisfactory degree, depending upon the kind of image to be recorded.
This problem will be explained taking as an example an apparatus in which four ink-jet heads of cyan, magenta, yellow and black are arranged in this order. According to the results of examinations conducted by the present applicant, when recording in different colors superimposed one upon the other, beading is least conspicuous when magenta and yellow are superimposed one upon the other. Thus, it is desirable to combine magenta and yellow so that they may be ejected in the same scanning pass. Further, in reproduce the color red, magenta and yellow colors are superimposed one upon the other. In the above combination, however, these two colors are ejected during the same scan, so that the effect to be obtained by the double pass scanning cannot be expected.
In this way, the combination providing the maximum effect is determined by the combination of inks ejected at each scanning, which is set in advance, so that, in some cases, the effect of the double pass scanning cannot be obtained at all, depending upon the kind of image to be recorded.
The above problem is more conspicuous in the construction described below: for example, in a copying machine serving as a color image recording apparatus, it is becoming general practice to provide a mono-color mode in which the entire original is copied in a designated color. In this case also, the effect of the double pass scanning may not be obtained depending upon the color designated in the mono-color mode.
The colors that can be selected in the mono-color mode are the seven colors of cyan, magenta, yellow, black, red, green and blue. Of these colors, when, for example, red is selected for mono-color recording, the above-described effect of the double pass scanning cannot be obtained.